In a Fibre Channel-to-SCSI router architecture, one SCSI initiator (interface), such as a SCSI router, can speak to multiple SCSI targets on behalf of many Fibre Channel (“FC”) initiators (hosts). The SCSI router serves as an interface to efficiently manage commands and communications between the FC initiators and the SCSI targets. On the SCSI side of the network, each of the SCSI targets is only aware of the SCSI router interface connection to which it is directly attached, and likewise each of the FC hosts sees only the SCSI router as a target. Neither the FC hosts nor the SCSI targets are aware of targets or initiators, respectively, on the other side of the SCSI router to which they are connected.
In a Fibre Channel-to-SCSI network, a SCSI router provides a pass-through data management role. For example, when a Fibre Channel host issues a command to a SCSI target the SCSI router receives the command and forwards it to the target. To the FC host, the SCSI router is the target, and the data management role provided by the SCSI router is transparent to the host. Similarly the SCSI target on the other side of the SCSI router sees the SCSI router to which it is attached as the initiator of the command. The data management role provided by the SCSI router is likewise transparent to the SCSI target.
As part of their data management role, SCSI routers in a Fibre Channel-to-SCSI network receive read and write commands from FC hosts. The amount of data contained in these read/write commands can be more than the capacity of the memory in the SCSI router. For example, a write command might consist of a one megabyte (“meg”) stream of data while the SCSI router may have only a half meg worth of memory buffers to receive, store and forward the write command.
In a typical existing SCSI router implementation, the SCSI router saves the data and routes it to the target device. If the target device is a sequential access device, such as a tape backup, this can result in rejection of the command and possible corruption of the data if the available memory in the SCSI router is insufficient to capture all the data. Existing router architectures typically have no mechanism for dealing with this situation. Existing SCSI routers likewise have no mechanism available for informing the FC host of the available memory size prior to the host issuing the command. Although there are some existing SCSI router implementations that provide for initial discovery of available memory by the FC host, these methods are inefficient, slow and expensive to implement.
Existing systems and methods for managing data flow in a Fibre Channel-to-SCSI network typically store all the data associated with a command in the SCSI router memory before forwarding the data to the SCSI target device. Once the storing event is complete, the write or read to the SCSI target is performed. Therefore, even when there is enough physical memory present to handle a read or write command, current systems provide relatively low performance and efficiency because available memory buffers are tied up until the read or write command is complete. These memory buffers cannot be used by other FC hosts until the current read or write command has completed execution. Current systems therefore both tie-up available memory resources for a longer time than necessary and limit Fibre Channel network performance, even in situations where the SCSI router memory is enough to handle the data associated with a read or write command.
In a Fibre Channel-to-SCSI network the possibility also exists that a SCSI target may be incapable of receiving data at the same rate that a FC host is providing the data. For example, during a write event a Fibre Channel host may provide a larger data stream than the intended SCSI target is currently capable of receiving. In such a case, the SCSI router interface attempts to match the data transfer rates between the Fibre Channel host and the SCSI target or risks corruption and possible loss of data. In currently existing Fibre Channel-to-SCSI data management methods and systems if the available memory buffer space is insufficient to provide a delay to match the data rates between the FC host and the SCSI target, the command may be aborted and the data being transferred may be corrupted or lost. Currently existing systems and methods do not have the capability to hold-off the FC host until enough memory buffer space becomes available.
Additionally, because currently existing Fibre Channel-to-SCSI network data management systems and methods typically store an entire read or write command data before forwarding the data to or from the SCSI target, subsequent requests to use the memory cannot be satisfied until the prior command is complete. In a situation where sufficient memory is unavailable to process additional commands from a FC host (or from a different FC host), currently existing systems and methods reject the command and the command is lost unless it is reissued by the host.
In prior art systems, a read/write command requiring a data transfer larger than the available buffer memory size would typically get rejected. This could result not only in corruption or loss of data, but in a failure of the FC host or the SCSI target involved in the data transfer. This is not a good situation because it means that the SCSI router is incompatible with the FC host. Especially in the case of a sequential access target device, the backup or transfer in progress could fail and the target device might have to be reset. Prior art systems dealt with this problem by limiting the size of read/write commands. This solution resulted in reduced network performance.
Furthermore, in the case of a sequential access target device, it is not possible to break-up a write or read command into discrete pieces that can be written to the target at different physical locations. For example, if a FC host issues a 1,024K write command to a sequential access device, the sequential access device will write the data to a physically continuous 1,024K memory block. The sequential access target device expects to subsequently read the data as a 1024K continuous block with a single end-of-record indicator at the end of the read. If an end-of-record indicator were present anywhere else in the data, a sequential access device would fail and the operation aborted. For this reason, a read/write command to or from a sequential access device in a prior art system had to be stored completely in the available SCSI router memory and then transferred to the target device so as to be physically written in a continuous manner.
Prior art Fibre Channel-to-SCSI data management methods and systems implemented within a SCSI router also require larger amounts of memory to deal with the limitations inherent to sequential access devices. This results in a correspondingly higher cost.